Colloids are dispersion systems composed of a particulate-form dispersed substance and a continuous-phase dispersion medium. The dispersed substance and dispersion medium are not uniformly mixed. As a means of evaluating the form of this kind of non-uniform structure, methods are known for measuring its electrical properties, such as conductivity, dielectric constant, etc. In recent years, methods for measuring the dielectric constant by using impedance measurements have been studied. For example, the electrical impedance of food colloids shows somewhat greater electrostatic capacitance than pure water. Means have been developed for detecting the structure of a colloid (dimensions of the particles of the dispersed substance, their density, etc.) from this electrostatic capacity and their frequency properties (frequency distribution).
If an electrostatic capacity measurement, i.e., a dielectric constant measurement, can be practically achieved, a large contribution will have been made to the study of the stabilization of dispersion systems and product quality control.
Even though the advantages of electrostatic capacity measurements are understood, it is difficult in practice to expediently make such measurements. The first reason is that, since (especially with foods) the source is a biological organism, the colloid solutions are aqueous, contain large quantities of ions, have large conductivities and small electrostatic capacities. For example, the magnitude of the susceptance (electrostatic capacitance) in comparison with the conductance of a certain fermentation solution is approximately 1/500. Nevertheless, the number of fermenting bacteria in the fermentation solution is correlated with its dielectric constant. Therefore, in order to control accurately the fermentation process by measuring electrostatic capacity, which varies with the progress of the fermentation, a resolution of approximately 1/5000 rad is necessary. The electrostatic capacity measurement values must be correctly separated and isolated so that they are not perturbed by changes in conductance. For this purpose, the phase angle of the complex impedance measurement must be correctly calibrated.
Even though a colloid solution (that is the subject of measurement) is enclosed between 2 electrodes and its electrostatic capacity is measured, such measurement is difficult because a low resistance is equivalently present in parallel with the electrostatic capacity.
The second reason why it is difficult to make electrostatic capacity measurements is a fundamental problem in electrochemistry. Because of the effect of interface polarization of the electrode plates, it is extremely difficult to isolate only the electrostatic capacity of the colloid solution. Interface polarization is primarily an electrostatic capacity effect resulting from a contact impedance produced between the solution and an electrode plate. If this interface polarization could be removed and only the impedance of the solution itself measured, quantitative information concerning the structure of the colloid could be obtained from the measurement's frequency distribution.
When a solution is measured with 2 electrodes, interface polarization is a problem. Since interface polarization is a type of contact impedance, it appears that it can be easily resolved by a 4-terminal measurement (4-terminal method). Such an operation will be discussed in conjunction with FIG. 10 to describe the principle of a 4-terminal measurement. Electrodes 61 and 64 supply current and are immersed in a colloid solution 21 filling a solution tank 20. A signal is applied between electrodes 61 and 64 from signal source 2 and an alternating current is caused to flow into solution 21; the amplitude and phase of the current are measured by ammeter 5. Measurement electrodes 62 and 63 are immersed in solution 21 between electrodes 61 and 64, and the amplitude and phase of the voltage between them is measured by voltmeter 4. It should be possible to obtain the impedance from the ratio of the measured values of voltage and current.
However, even though a theoretical voltmeter with an infinitely large input impedance is used for voltmeter 4, only a small improvement in the interface polarization problem results. The reasons for this are as follows. First, the electrical field produced by the voltage applied between electrodes 61 and 64 must be equipotential on planes parallel to the electrode plates, and potential-measuring electrodes 62 and 63 must be inserted on the respective equipotential planes. It is extremely difficult to practically achieve this arrangement. If they are misaligned, the potentials are different at positions in the longitudinal direction of potential measurement electrodes 62 and 63, and excesses and deficits of current are produced in the electrodes. Moreover, since potential-measuring electrodes 62, 63 have finite thicknesses, part of the solution will be short-circuited, and current excesses and deficits will result. The excesses and deficits of currents passing through the interface between solution 21 and electrodes 61, 64 produce interface polarization. As a result, an interface polarization error is introduced into the calculated impedance value (a ratio of voltage and current).
FIG. 11 is a 4-electrode method that reduces the effect of the interface polarization of FIG. 10. In this figure, the same reference numbers are assigned to the elements which have the same functions as in FIG. 10. The narrow parts of solution tank 20 function like liquid electrodes; they touch the solution columns that are the objects of measurement, without interface polarization. However, as can be predicted, reducing the effects of the interface polarization and trying to measure the solution columns accurately are not necessary compatible. The extension of part of the liquid polarization places strict requirements on the large input impedance, obtained at the voltmeter 4, which is a differential potential difference detector, and requires common mode signal removal. The correction procedure for obtaining high measurement accuracies over a wide frequency range and a wide dielectric constant range becomes complex. In addition, of course, there are inconveniences of operation, such as cleaning.
In order to fundamentally solve the problem of the effects of interface polarization, a measurement technique that requires no electrodes has been proposed, (i.e., eliminating the contact between the electrodes and the solution). In such method, a closed-circuit current is allowed to flow in the solution by electromagnetic induction, and the current is measured by electromagnetic induction. FIG. 12 is a diagram which illustrates the principles of this measurement technique.
A primary transformer, consists of a primary coil 11 wound on a toroidal core 10, and a secondary transformer, consists of a secondary coil 13 wound on a toroidal core 12. When the transformers are immersed in solution 21, the solution completes a circuit therebetween. Therefore, when an alternating current is allowed to flow to primary coil 11 from signal source 2, a closed circuit current 65 is caused to flow in the solution by electromagnetic induction. Due to this closed circuit current 65, an alternating current magnetic flux is produced in toroidal core 12, and an electromagnetic force is produced in secondary coil 13, causing a current to flow. Therefore, if the voltage induced in secondary coil 13 or the current flowing in secondary coil 13 is measured, the closed circuit current is obtained. Since the magnitude of the closed circuit current produced by the electromagnetic induction is proportional to the admittance of solution 21, the dielectric constant can be calculated from the measured value of the closed circuit current. This method does not produce interface polarization, since electrodes are not used as the source of current flowing in the solution.
Therefore, the electromagnetic induction method is primarily a method for measuring solutions with high conductivity; however, it is not a practical method for systems with small dielectric constants, i.e., for the measurement of electrostatic capacities. This is because, when a detection device such as shown in FIG. 12 is used to measure electrostatic capacities, measurement errors are produced by undesired coupling. This undesired coupling is shown in FIG. 13. If there is a coupling between primary coil 11 and secondary coil 13, due to electrostatic capacity 66, a current (other than the closed circuit current due to the original electromagnetic induction) will flow in the secondary coil, introducing an error. Since this electrostatic capacity 66 varies with the solution, it is impossible to compensate for it. Moreover, an error is also produced by the dielectric coupling between the coils due to the stray magnetic flux 67 from the coils. For these reasons, it has not been possible to measure susceptances that are small, in proportion to the conductances of solutions.